The present invention broadly relates to gas detectors and, more specifically, pertains to a new and improved arrangement for detecting a foreign component contained in a gas and comprising a measurement chamber accessible to the gas to be investigated, a sensor generating an output signal in dependence of the foreign component content of the gas being investigated, and an electrical circuit for evaluating the output signal of the sensor. The invention also pertains to applications of the inventive arrangement.
Such an arrangement can, in principal, be employed for detecting any random solid, liquid or gaseous foreign components in a gas, in particular gas-born particles or aerosols. The sensor is then tuned or adapted to the foreign components and can, for instance, be constructed as a transmission measurement path exploiting the diminution or extinction of electromagnetic radiation. The employment of the arrangement for detecting incendiary aerosols or smoke particles in air, for instance for the purpose of incendiary or fire reporting, is of particular utility.
Hitherto, scattered radiation detectors in which the electromagnetic radiation scattered by smoke particles, i.e. visible light or infrared radiation, was registered by a scattered radiation receiver arranged outside the direct radiation path or beam were often employed for the detection of incendiary aerosols or smoke. Such a detector can be adjusted to be very sensitive, since in the absence of smoke there is no scattered radiation, i.e. the signal is nearly zero, and therefore in the presence of smoke only a small deviation from the zero value must be detected, which presents no great difficulties in measurement technology. It is, however, disadvantageous that such smoke detectors only react to radiation-scattering smoke, for instance to white smoke of high water vapor content, but react very little or not at all to black, preferentially radiation-absorptive smoke which scatters little or not at all.
In order to avoid this disadvantage and to simultaneously detect both radiation-scattering and radiation-absorbing smoke, it has proven advantageous to employ the transmission variability of a gas, for instance air, as a criterion for the presence of aerosols. The radiation extinction or diminution of a radiation beam transmitted by a source of radiation so as to traverse a transmission measurement path and picked up by a radiation receiver is evaluated to detect smoke. Since a relatively small deviation from a large standard value must be detected for this purpose, a relatively great length of the transmission measurement path is necessary for reliable detection of small smoke concentrations which, in practice, lies in the one meter range, a fact which considerably inhibits practical application and renders practical application nearly impossible without further measures.
In order to overcome this disadvantage and to avoid such large dimensions, it is known to employ a folded radiation beam or path in which the radiation is deflected by reflectors or mirrors so that, with a sufficient number of reflectors, the dimensions of the smoke detector can be reduced to a practically acceptable value in the ten centimeter (10 cm) range. In this case, however, the problem arises that radiation extinction or diminution of the type due to smoke can be counterfeited or mimicked by a reduction of reflectivity of the mirrors, for instance due to dust on the surfaces, as well as by aging of the source of radiation, and false alarms can be triggered. The smoke detector described in the German Patent Publication No. 3,117,757 attempts to avoid this disadvantage of gradual dust collection on the optics by employing a special stabilizing and regulating electronic circuit, so that gradual changes are compensated and a signal is only generated in response to rapid changes. However, a considerable amount of circuitry is required and furthermore a gradual development of smoke cannot be differentiated from a gradual collection of dust and can therefore not be recognized and detected.
Alternatively, smoke detectors have been variously proposed, e.g. in the German Pat. No. 1,038,454 and the Swiss Pat. No. 561,942 which comprise, in addition to the transmission measurement path itself, a reference measurement path having a different degree of radiation extinction or diminution or a different optical path length. Since, however, a very small difference between two large signals must be formed, a considerable amount of circuitry for electrical stabilization is required. Furthermore, precise mechanical adjustment and corresponding maintenance over long periods of time are required since such smoke detectors tend to mechanical instability and vibrational sensitivity.
In the smoke concentration measuring device described in the German Patent Publication No. 1,942,942 a reference beam path is renounced in order to avoid the disadvantages of a twin-beam method and, instead, the path length of the transmission measurement path is mechanically modulated, for instance by oscillating or vibrating a transparent termination window of the transmission measurement chamber or another component such as the radiation source, the radiation receiver or a reflector at a prescribed amplitude so that the optical path length varies between two values. The variable portion of the receiver output signal is an indication of the smoke density or concentration.
Such length modulation, however, only comprises a small portion of the total length of the transmission measurement path. In practice, for instance in incendiary or fire reporting applications, quite large modulation amplitudes lying at least in the centimeter range are necessary for achieving a sufficient smoke sensitivity. The reliability and the energy consumption of an oscillatory or vibratory generator system for such great amplitudes is, however, not compatible with the requirements of long term, maintenance-free and energy-saving operation for smoke detectors.
The smoke detection arrangement disclosed in the U.S. Pat. No. 2,486,622 takes a different direction in which the gas to be investigated is transported through a measurement chamber alternatingly directly and indirectly through a smoke-absorbing filter by means of a blower and a controllable valve. The variable portion of the output signal of the sensor constructed as a transmission measurement path or scattered light detector is an indication of the smoke concentration. It is, however, a disadvantage that the blower motor must be constantly in operation and must continuously deliver a considerable amount of air. Reliable, maintenance-free and energy-saving long term operation is therefore also not assured. In addition, the necessary filter can only absorb a certain quantity of aerosols, smoke or dust particles. Low foreign particle concentrations always present in the air therefore cumulate rapidly on the filter and render the latter ineffective in a short period of time, so that in an alarm situation, for instance when a fire breaks out, the smoke generated is not detected unless the filter is exchanged, cleaned or regenerated at short time intervals.